Liquid velocity in a high-solids-loading three-phase external-loop airlift reactor

Abstract

BACKGROUND: Airlift reactors are of interest for many different processes, especially for three-phase systems. In this study the behavior of a high-loading three-phase external-loop airlift reactor was examined. In particular, the effect of parameters such as airflow rate (riser superficial gas velocities between 0.003 and 0.017 m s-1), solids loading (up to 50%, v/v) on liquid circulation velocity in the air-water-alginate beads system as a crucial hydrodynamic parameter was studied. RESULTS: It was observed that increase of the airflow rate resulted in increase of the liquid velocity in the system. The same result but less pronounced was observed by introducing small amounts of solid particles up to 7.5% v/v. However, further introduction of solids caused decrease of the liquid velocity. Laminar regime for the liquid circulation was observed for low gas velocities. Minimum gas velocities for recirculation initiation in the reactor were determined for all solid loadings and linear dependence on the solid content was found. Gas holdups for the three-phase system were larger than for the two-phase system in all experiments. A simple model for predicting the liquid circulation velocity in the three-phase system with high solid loading of low-density particles was developed. This model is based on the viscosity of integrated medium (solid + liquid) which is a new aspect to analyze this phenomenon. CONCLUSIONS: The developed model shows very good agreement with the experimental results for all solid loadings. It also includes the influence of reactor geometry on the liquid circulation velocity thus enabling optimization.